To perform a seismic marine acquisition in a survey area, it is common to use seismic sources (guns, vibratory sources, . . . ) and seismic sensors. The sensors are housed in cables, called streamers or linear antennas. Several streamers are used together to form an array of thousands of sensors. Sources and streamers are both towed by vessels. A vessel tows generally one or more streamers and can be equipped (or not) with seismic sources.
To collect the geophysical data in the marine environment, one or several immerged seismic sources towed by at least one shooter vessel are activated to generate single pulses or continuous sweep of energy. The signals generated by each source travels through the layers of the earth crust and the reflected signals are captured by the sensors (hydrophones) in the streamers towed by at least one listener vessel.
The data collected by all sensors during a period of a few seconds (called record length) is then stored by a recording system as a dataset (usually a file in SEG-D format). The interpretation of the seismic data contained in the SEG-D files is used to compute a 3D image of the earth crust.
The theoretical position of seismic sources and seismic sensors for each acquisition is described in a specific document called “preplot”: the more the preplot requirements are respected, the more the quality imaging of the earth crust is. The actual positions of all equipments (hydrophones and guns) are known thanks to well-known measure means (GPS, RGPS, acoustics, compasses, depth sensors . . . ).
The acquisition process is controlled and monitored by an Integrated Navigation System (INS), whose role is to compute position of sensors and sources, drive vessels along their acquisition path, according to the preplot geometry, and to activate sources to perform seismic acquisition at desired location.
This time and space synchronization (between sources and sensors) is achieved by the exchange of space events (called bull's eye), giving position at which each vessel should be (this giving position of sensors and sources), and time events (called shots), giving time at which a source is to be activated.
To further increase the quality of seismic imaging, the seismic surveys are now performed in multi-vessel operation, in order to obtain a wide azimuth illumination of the earth's crust. In this case, an initiating pulse is transmitted via a radio modem line to the vessel or vessels participating in the survey. In multi-vessel operation, a known solution is to centralize the shooting management on a particular vessel called master vessel. For example, this master vessel tows a plurality of seismic streamers and also tows one or more seismic sources (guns for example).
All vessels position information are sent to this master vessel, which in return generates space and time events for all vessels, according to preplot. The flow of information is exchanged in real time through wireless channels, which are not 100% reliable due to fading, long distances between vessels, multipath and floating obstructions.
Referring to FIG. 1, an example of such a multi-vessel marine seismic acquisition is schematically shown.
As explained above, for the sake of simplicity, we consider the simple case of a couple of vessels including a shooter vessel V2, which tows a source G2 (for example a gun), and a listener vessel V1, which tows a plurality of streamers S1. We also assume that the listener vessel V1 is the master vessel and the shooter vessel V2 is the slave vessel. The master vessel V1 is moving in parallel with the slave vessel V2. Streamers S1 include seismic signal receiving sensors (for example, hydrophones), which receive reflections of signals from source G2.
Source G2 is controlled by a source controller located on slave vessel V2. Signals received by streamers S1 are recorded onboard master vessel V1 by a seismic recorder.
It is desirable to record reflections of signals initiated by source G2 at streamers S1. For this purpose, synchronization of the recording system on the listener vessel (master vessel in this example) with the source on the shooter vessel (slave vessel in this example) is critical to accurate data acquisition. However, while a recording system on any given vessel is accurately synchronized with a source on the same vessel, precise coordination of a recording system and a source located on a listener vessel and a shooter vessel respectively has proved to be difficult.
An example of the problem occurs when the recording system on the master vessel is set to record seismic reflections from a source on a slave vessel. The master vessel transmits a shoot command to the slave vessel's seismic source. There is a small, but significant, delay between the instant when the master vessel computer issues the shoot command for the slave vessel source, and the instant when the slave vessel actually causes the shot. This delay is caused by the delay inherent in the computers, radio transmission, and receiving links between the vessels.
The instant in time when any source actually fires, and the instant when any particular reflection is received by a streamer, are termed “events” which must be synchronized. Those of skill in the art will also recognize that synchronization among and between other events is also of critical importance in multi-vessel seismic exploration. Examples of such other events include: the instant in time when a particular vessel crosses a particular point on the seabed floor, the instant in time when a seismic source on a particular vessel is initiated, etc.
According to typical systems, a VHF radio link is used to communicate the events between the two vessels (master and slave), with, for example, a phase locked loop (PLL) circuit used to detect the events communicated on the radio link. “Fire” and “time break” commands are generated across the radio link at specific instants, based upon the calculated delay, which will, hopefully, cause the recorder to begin recording at about the same instant as the firing of the source. However, such a system requires a constantly operational radio transmission link, and the system also requires regular calibration. Calibration is normally carried out “off-line,” the result of which is that timing errors may occur between calibrations, these timing errors being undetected.
During seismic survey, the radio link between two vessels (or more) can be lost or down (broken), for example when the two vessels sail on both sides of a metallic barrier, such as in an offshore platform.
If radio link is lost or down at a time when a shoot command is transmitted to the slave vessel's source, the shoot command will not be received, the shot will not be made, and the vessels will pass by a spot where data is required.
Thus, such loss of wireless communication (radio link) prevents master vessel to know precisely position of other vessels and/or prevents other vessels to receive time and space events, thus making shot imprecise in time and/or space or missing. The consequence is an altered image of the earth's crust.